I. Field of the Invention
This invention relates to an improved process of surface sizing paper and paperboard utilizing a starch blend of hydrophobic and non-hydrophobic starches. In a further aspect, the invention relates to the sized paper product as well as to the novel size blend.
As used herein, the terms "paper and paperboard" include sheet-like masses and molded products made from fibrous cellulosic materials which may be derived from both natural and synthetic sources. Also included are sheet-like masses and molded products prepared from combinations of cellulosic and non-cellulosic materials derived from synthetics such as polyamide, polyester and polyacrylic resin fibers as well as from mineral fibers such as asbestos and glass.
II. Brief Description of the Prior Art
Paper and paperboard are often sized with various materials for the purpose of increasing their strength, their resistance to picking and scuffing, and their resistance to undue penetration of water, organic solvents, oils, inks and various types of aqueous solutions as well as for the purpose of improving their smoothness and optical characteristics. When sizing materials are applied to the surface of a web or sheet in order to cement the surface fibers to the body of the paper and to modify the sheet surface, the process is known as external or surface sizing; the latter process being quite distinct from an internal sizing process wherein sizing agents are admixed with the pulp slurry prior to its being converted into web or sheet form.
Among various materials which have been utilized as surface sizing agents are included conventional and modified starches, polyvinyl alcohol, cellulosic derivatives, gelatin, rosin, proteins such as casein, and synthetic polymers. Although these materials are effective to various degrees under certain conditions, their use is nonetheless subject to one or more limitations. For example, it is often necessary to utilize high concentration levels of such sizes in order to achieve the desired strength and aqueous holdout characteristics. Since it is known that the opacity and brightness of the base paper sheet decrease in proportion to the amount of size that is applied thereto, a direct result of the required use of such high concentration levels is a reduction in the optical properties of the treated paper. Furthermore, the use of such high concentration levels makes the sizing of specialty papers economically unattractive inasmuch as high cost, quality sizes, e.g. gelatin, animal glue and casein, are usually utilized for such purposes. In addition, certain prior art sizing agents impart relatively poor water resistance and must be used in conjunction with insolubilizing agents in order that satisfactory water resistance is obtained.
It is the prime object of this invention to provide improved starch based surface sizing agents whose use results in the preparation of paper which is characterized by improved water resistance and other size properties. A further object of this invention involves the use of surface sizing agents which may be employed with all types of paper fiber. Various other objects and advantages of this invention will become apparent to the practitioner from the following detailed description.